Container molding techniques have encouraged the use of light-weight containers in order to save on material costs and to obtain faster molding cycles. Naturally, as the containers become lighter in weight, the wall thickness diminishes and problems are encountered in the rapid ejection from the molds.
U.S. Pat. Nos. 4,179,254 and 4,438,065 show conventional methods for ejecting cup-shaped workpieces with closed front ends using stripper rings supplemented by venting holes, slots or air valves in the mold core to break the vacuum created between the core and the molded part or workpiece during ejection and to prevent collapsing of the workpiece.
The use of stripper rings to eject the container by pressing on its rim requires that the container be reasonably stiff so that it can be stripped off the core without buckling or even "folding over itself" like the finger of a tight-fitting glove during removal.
However, the demand for such stiffness to facilitate removal of the container from the core implies greater wall thickness, stiffer rather than easy-flowing materials, and longer cooling time. Any one of these factors contributes to slower cycles and higher costs.
An obvious answer to this difficulty, and one often attempted, has been to increase the air supply through the air vents or valves within the core designed to "blow" the workpieces off the core. This is sometimes successful especially with shallow workpieces and with strongly tapered ones that have a significant difference between top and bottom diameters. However, this method has not been satisfactory for deeper containers, particularly when their peripheral walls are nearly cylindrical, i.e. with a very small draft angle. In such cases, the air pressure inside the container tends to stretch its closed front end or bottom and thereby tighten the grip of the container on the core. The plastic may even burst at the cup bottom.
The use of vents or valves within the core is subject to other disadvantages, some of which are referred to in U.S. Pat. No. 4,438,065 and include the Venturi effect caused by the air escaping at the cup bottom and blowing out through the gap between the core and workpiece, problems with multi-cavity molds and other difficulties.
A significant disadvantage in the use of vents or valves relates to the problem of fixed vents. In order to prevent the plastic resin from entering and blocking the vents during the injection segment of the molding cycle, it is necessary to make the size of the fixed vents very small, in the order of 0.0005" to 0.001". Not only does this require extremely accurate and expensive machining of the component parts, but during the operation of the mold the air which will issue from the vents must first be filtered to prevent blocking of the vents by foreign particles carried through the mold to the vent. Naturally, filtering of the air is an operation of additional expense.
Accordingly, it is a principal object of the present invention to provide an improved mold core for ejecting a cup-shaped workpiece.
It is a further object of the present invention to provide an improved mold core as aforesaid which permits rapid ejection of workpieces, which does not require extremely accurate and expensive machining and does not incur the risk of preferential fill or vent blocking with its attendant problems.
Further objects and advantages of the present invention will appear hereinbelow.